This invention relates to electrostatic particle accelerators and chiefly relates to evacuated accelerator tube assemblies for use in such accelerators. The accelerator tube assembly currently favoured comprises a stack of rings of insulating material eg glass or ceramic interleaved with and bonded to, annular metal electrodes, termed herein intermediate electrodes which, being located between a high voltage terminal and ground, are held at graded potentials by means of electrical resistance bridges (or otherwise) connected as potential dividers. Once a high voltage is applied to the terminal an electric field is established along the bore of the tube, the field being axially of the tube, and the potentials along the bore constitute a potential gradient down which particles may be induced to pass and to be accelerated in the process to high energies. In any accelerator tube assembly there may be found charged particles which may defeat efforts to produce a high efficiency tube by various mechanisms. As will be known, in the prior designs these unwanted particles have been taken out of the system by magnets or by trapping on specially provided electrodes which are reverse biased. Again the axial drift of these particles has been limited by employing narrowly apertured diaphragms. There remains yet still scope for improvement, however, and it is an object of the present invention to reduce the effect that these unwanted particles may have on tube efficiency.
More particularly the inventors have considered the effects on tube efficiency of randomly charged particles, or their successors, possibly being accelerated in counter current to the intended direction for a main particle beam, possibly with consequential scattering, and also of charge build up on the inuslator rings following collisions between particles and the tube structure.